Conventionally, a configuration which measures an input current to an in-vehicle electric motor and changes a rotation speed of the electric motor based on the measurement result has been proposed by, for example, Patent Document 1. In general, an inverter receives a command rotation speed from a higher level ECU and operates to control an actual rotation speed to follow the command rotation speed.
The higher level ECU and the inverter communicate with each other by an arbitrary communication protocol. The command rotation speed is updated at a regular interval. The electric motor stores a constant accelerated rate. The electric motor accelerates its rotation speed with the accelerated rate for every update of the command rotation speed, and terminates the acceleration when the actual rotation speed reaches the command rotation speed. Accordingly, the operation of the electric motor becomes constant-speed rotation.
When the update interval of the command rotation speed is set at 1 second, for example, and a variation width of the command rotation speed is an increment less than the constant accelerated rate, the acceleration of the electric motor is terminated during the update interval. Thus, the electric motor continues to rotate at the constant speed until next update of the command rotation speed. When the increment of the command rotation speed which is less than the constant accelerated rate occurs every 1 second successively, the operation of the electric motor alternates between the acceleration and the constant-speed rotation. As a result, a noise like a musical scale may be generated from the electric motor and provide a user with a feeling of strangeness.
Similarly, when the electric motor is decelerated, the operation of the electric motor alternates between deceleration and constant-speed rotation. Hence, a noise similar to the above-described noise may generate also when the electric motor is decelerated.